The invention generally relates to the formation of thin gold films, and relates in particular the forming of thin gold films using electroless plating techniques.
Gold films, and in particular, thin gold films have widespread technological utility, from forming conductive elements and overlayers, to serving as a platform for chemical surface modification by molecular self-assembly. For gold films incorporated into conventional micro- and nanofabricated devices, silicon nitride is an appealing choice for a substrate. It is a standard nanofabrication material, offering, in addition, favorable inherent properties such as mechanical strength, chemical resistance, and dielectric strength. Silicon nitride is thus ubiquitous as a structural and functional element in nanofabricated devices, where it plays a variety of roles.
The surface chemistry of silicon nitride, however, presents special challenges given the complex mixture of silicon-, oxygen-, and nitrogen-bearing surface species. The nominal surface modification of silicon nitride is frequently carried out in practice using silane-based modification of a silica layer that may itself not be well-defined. Thus, there remains both a need and opportunity to expand the suite of approaches useful for surface functionalizing silicon nitride directly.
Electroless deposition is a particularly compelling approach to film formation for a variety of reasons. For one, deposition proceeds from solution allowing the coating of three-dimensional surfaces, including surfaces hidden from line-of-sight deposition methods. Also, no electrochemical instrumentation is required; no electrical power must be supplied nor must the substrate be conductive; there is no need for expensive vacuum deposition equipment. Further, a variety of classical physicochemical parameters such as reagent composition, solution properties such as pH and viscosity, and temperature, are available to tune the film properties.
There are many approaches for the electroless plating of substrates such as polymers, for example, but no established techniques for the direct metal-cation-mediated electroless plating of gold onto silicon nitride. One interesting sequence exists for the electroless gold plating of poly(vinylpyrrolidone)-coated polycarbonate substrates (Au/PVP): direct sensitization of the PVP surface with Sn2+, activation by immersion in ammoniacal silver nitrate to oxidize the surface Sn2+ to Sn4+ by reducing Ag+ to elemental silver (producing, also, a small amount of silver oxide), and finally gold plating by galvanic displacement of the silver with reduction of Au(I) to Au(0) accompanied by the oxidation of formaldehyde. Amine and carbonyl groups in the PVP layer were proposed to complex the tin cation during sensitization.
Extending this approach, Sn2+ has been reported to complex effectively with oxygen-rich polymer surfaces and with quartz and silica substrates. Tin(II) sensitization has also been reported on NaOH-roughened surfaces, suggesting that a specific chemical interaction may not be essential, and underscoring the utility of electroless plating for rough and high-surface-area surfaces where physical deposition is challenged.
In principle, though, a smooth silicon nitride substrate with a well-defined silica surface layer should be amenable to direct tin sensitization. Electroless deposition of gold on planar silicon nitride however, has been limited to routes requiring the use of a silica layer with organic linkers and metal layers between the silicon nitride and gold overlayer. In the first case, covalent attachment of an organic monolayer using silane chemistry can be beneficial for film adhesion, but adds operational complexity and can constrain downstream processing conditions. In the second case, the intervening layers may lend beneficial properties, or may similarly introduce compositional constraints on applications, or morphological constraints on the final gold film nanostructure. Regardless of the ability to carry out a silica-based modification, it does not eliminate the benefits of a direct functionalization of silicon nitride.
There remains a need therefore, for an improved method for providing plating of gold onto silicon nitride thin films without the above discussed shortcomings.